1. Field of the Invention
The present invention relates generally to measuring instruments and, more particularly, to a method and apparatus for measuring the thickness of wet coatings or films.
2. Description of the Prior Art
It is often desired to determine the thickness of a wet film applied to a surface in order to check the accuracy of a coating operation being employed to apply the coating. Examples of devices used to make such measurements can be found in U.S. Pat. No. 2,507,592 and U.S. Pat. No. 3,128,558, both issued to Maynard R. Euverard.
Such gauges are designed to roll along a film-covered surface in order to measure the thickness of the film. The structure employed in the known devices to carry out the measurement includes a pair of surface-contacting support disks which are mounted on a rotary axle. Between the support disks, or positioned to the outside of one of them, is a thickness measuring structure which can be in the form of an eccentrically mounted disk. The measuring disk presents a continuous surface located at different distances from the outer diameter of the support disks. By rolling the gauge across the film covered surface, a portion of the surface of the measuring disk, situated above the level of the film, will not be wetted, while those surfaces at or below such level will be wetted. The operator need only thereafter inspect the gauge and take the reading of that wetted surface located the greatest distance from the plane of the periphery of the support disks.
Although this known type of measuring gauge permits very accurate measurements to be made within the range of the gauge, as the gauge is usually produced from a single solid block of metal, the range itself is typically very limited to enhance the gauge accuracy and may be limited so as to cover as small a range as 0 to 2 mils (0.000 to 0.002 inches).
Another incidental effect of using the known gauges is that a mar is usually left in most types of films by the rolling edges of the support disks which contact the film during the entire rolling measurement operation.
It is desired to provide a gauge which permits accurate measurements to be made of wet coatings and films having an increased range of thicknesses, e.g. up to 200 mils and even greater, with the same relative accuracy as the known devices. It is further desired to provide a gauge which can be used with less risk of substantially interfering with the coating during the measuring operation.